This invention relates to a current limiting circuit and more particularly a circuit which limits the amount of charge flowing in a photodetector such as an avalanche photodiode to prevent damage thereof and also to prevent damage to any front end sensing electronics.
A photodetector is a device that converts light intensity into an electrical signal. The three most common types of photodetectors are photodiodes, avalanche photodiodes, and photomultiplier tubes. The first two types of photodetectors are semiconductor devices that detect low levels of electromagnetic radiation (photons) and are constructed so that a photon dislodges an electron (primary electron) and creates a hole-electron pair. These holes and electrons move in the opposite direction in the semiconductor device due to the electrical field that is applied across the photodiode. This movement of electrons through the structure is called photocurrent and it is proportional to the light intensity. In avalanche photodiodes, the primary electron hits other atoms with sufficient. velocity and energy in the lattice structure to create additional electron-hole pairs. This cascade effect in avalanche photodiodes results in an effective gain and allows the detection of very low light levels. Indeed, single photon detection is possible using an avalanche photodiode combined with an active quench circuit. This type of operation of an avalanche photodiode is called the Geiger mode when the avalanche photodiode is biased above its breakdown voltage. Three examples of active quench circuits are shown in U.S. Pat. Nos. 5,532,474; 5,933,042; and 4,945,277 all of which are incorporated herein by this reference.
A photomultiplier tube type photodetector includes a cascade of dynodes in a vacuum tube that converts photons into electrons and the multiplication effect of the primary photon electron creates the necessary electrical gain.
Under normal operating conditions, photodetectors are biased below the breakdown voltage and the photocurrent is relatively small. In most of these applications, front end sensing electronics including, inter alia, an amplifier circuit is required to convert the photocurrent of the photodetector to a voltage level above the background noise of the following stage electronics. The resulting voltage signal is indicative of the light intensity striking the photodetector.
In all photodetector applications, damage would occur if the photocurrent induced by errant light exceeds the front end sensing electronics limited breakdown level. Errant light can strike the photodetector, for example, when a laser is in close proximity to the photodetector and laser light scatters and is directed at the photodetector. Decreasing the susceptibility of the front end sensing electronics to high photocurrent values is not an easy task and most of the time it leads to performance degradation including noise and degraded speed. In any case, improving the front end sensing electronics does not protect the photodetector itself which can sustain only a limited photocurrent prior to permanent damage.
In many applications, like LIDAR, fluorescence, and particle sizing, a powerful laser is used in conjunction with one or more photodetectors. In these applications, errant back reflections of the laser light into the photodetector is frequently a problem that can saturate the photodetector causing permanent damage. It has been demonstrated in the laboratory that the threshold for permanent damage in a commercial active quench avalanche photodiode is around I million photons within one nanosecond causing a photodiode current in excess of twenty miliamps. Lasers used in conjunction with many photodetector applications generate light levels many orders of magnitude above this permanent damage threshold. Thus, the need to quickly stop the photodiode current surge induced by errant light laser is mandatory in low light level detection applications such as single photon detection to avoid destruction of the photodiode.
One prior art way to limit the photodiode current is to use a high ohm (e.g., a kilo-ohm or higher) resistor in series with the photodetector. This approach has several limitations. Due to Ohm""s law, the current in this high ohm resistor creates a voltage drop across the resistor even in the normal operating mode of the photodiode which reduces the bias voltage across the photodiode. This voltage drop decreases the photodetection efficiency of the photodiode since the light detection efficiency of the photodetector is directly related to the bias voltage across the detector. On the other hand, the use of a smaller resistor would not adequately protect the photodiode.
Also known in the art is a clipper circuit used to protect the front end sensing electronics against high photodetector currents caused by errant light sources. The clipper circuit, however, does not limit the photodetector current and thus does not protect the photodetector against high photodetector currents.
It is therefore an object of this invention to provide a current limiting watchdog circuit for photodetectors including photodiodes, avalanche photodiodes, and photomultiplier tubes.
It is a further object of this invention to provide a watchdog current limiting circuit which can withstand high errant light levels without degradation of the photodetector or the front end sensing circuitry.
It is a further object of this invention to provide a watchdog current limiting circuit which limits the current flowing in the photodetector to avoid its destruction.
It is a further object of this invention to provide a watchdog current limiting circuit which does not affect the detection efficiency of the photodetector in its normal operating region.
It is a further object of this invention to provide a watchdog current limiting circuit which does not affect the input impedance of the front end sensing electronics attached to the output of the photodetector.
It is a further object of this invention to provide a watchdog current limiting circuit which operates independently of the supply voltage level applied to the photodetector.
This invention results from the realization that by connecting the source of a transistor to a sensing resistor which monitors the photodetector current and connecting the drain of the transistor to the photodetector, then the drain/current resistance of the transistor can be increased to the transistor""s saturation point to protect the photodetector from high currents caused by errant light sources but only when the photodetector current reaches a trigger point to thus maintain the detection efficiency of the photodetector below the trigger point where the drain/source resistance of the transistor is very low. In the preferred embodiment, the gate of the transistor is connected to a bias voltage so that the transistor gate/source voltage decreases to drive the transistor drain/source resistance up as the photodetector current increases. Also in the preferred embodiment, the bias voltage is independent of the polarization voltage applied to the photodetector.
This invention features a large current watchdog circuit for a photodetector. The watchdog circuit comprises a current sensing device responsive to current flowing through the photodetector; and a variable impedance element responsive to the current sensing device and the photodetector which increases in resistance in response to current flowing through the photodetector to protect the photodetector from high current levels.
In one example, the photodetector is a photodiode. One current sensing device is a resistor connected between the voltage supply source and the photodetector. In the same example, the variable impedance element is a transistor having its drain connected to the photodetector and its source connected to the resistor thus providing a drain/source resistance which varies in response to the current flowing through the photodetector. The gate of the transistor is connected to a bias voltage source. The bias voltage source may include a capacitor for holding the voltage at the gate of the transistor steady and a resistor in parallel with the capacitor. In the preferred embodiment, the bias voltage source draws voltage from the supply voltage source and includes a plurality of diodes interconnected between the supply voltage source and the gate of the transistor.
In another embodiment, the bias voltage source includes a Zener diode interconnected between the supply voltage source and the gate of the transistor. In still another example, the bias voltage source is a battery.
In the preferred embodiment, the watchdog circuit comprises a photodetector connected to a supply voltage source; a sensing resistor connected between the supply voltage source and the photodetector; a bias voltage source; and a transistor having its source connected to the sensing resistor, its drain connected to the photodetector, and its gate connected to the bias voltage source such that the drain/source resistance increases when the gate/source voltage decreases to thereby protect the photodetector from large currents.
In the broader sense, there is a photodetector connected to a voltage supply source; a variable impedance connected between the supply voltage source and the photodetector; and a variable voltage responsive to the photodetector current to drive the variable impedance up as the photodetector current increase. In the typical case, the variable impedance is the source and drain of a transistor connected between the voltage supply source and the photodetector. A resistor is then connected between the source of the transistor and the voltage supply source and the variable voltage is the gate and source of the transistor, the gate connected to a bias voltage source, the source connected between the photodetector and the resistor.
Also in the preferred embodiment, the watchdog circuit includes a photodetector connected to a voltage supply source; a bias voltage source; and a transistor having its source and drain interconnected between the photodetector and the voltage supply source and its gate connected to a bias source operating to keep the transistor on, the transistor having a low value source/drain resistance (e.g., 850 ohms) when the photodetector current is at nominal levels (e.g., 20 miliamps), so that the operation of the photodetector is not affected. The transistor becomes saturated, however, when the photodetector current exceeds the nominal levels, and then has a high source/drain resistance value (e.g., infinity) to protect the photodetector from high level current values. In one specific example, the transistor impedes photocurrent values above about 20 miliamps but has only an 85 ohm source/drain resistance when the photodocument is below 20 miliamps such that even avalanche photodiodes operated in the Geiger mode are not adversely affected by the watchdogs circuit when the photocurrent is below this nominal level.